For many years, conventional piping systems consisted solely of rigid, metallic pipes. Recently, however, plastic pipes have become popular because they are light, flexible, corrosion resistant, and can be installed with relative ease compared to metallic pipes. Despite these advantages, however, where rigidity is required plastic pipes have proven to be unacceptable substitutes for metallic pipes due to their flexibility.
For example, plastic pipes have not proven to be amenable to joining with conventional pipe couplings. When using push-on or mechanical-type couplings, the force necessary to form a gas or liquid-tight seal tends to deform the ends of the flexible pipe. In addition, pipes formed from flexible materials tend to cold-flow, relieving the sealing forces exerted from initial insertion and compromising the integrity of the joint. Also, when flexible plastic pipe is used in buried systems or the like, the end of the pipe is subjected to excessive pressure from the surrounding soil. This pressure can cause the end of the pipe to deform slightly, thereby causing a flow restriction within the pipe or a loss of sealing forces at a coupling.
To address these problems, attempts have been made to place inserts or stiffeners inside the flexible pipe at the end of the pipe segment, e.g. near the location of an intended joint. These stiffeners are made from a rigid material specifically sized to the minimum pipe inside diameter, and are typically press or wedge fit into the end of the pipe to provide an area of rigid support. This method makes the pipe stiff enough for couplings to work well, but has its limitations.
One problem which is inherent with the use of press or wedge fit pipe stiffeners is that their existence within the pipe causes a flow restriction. In areas where pressure drop and flow rates are critical, the presence of a stiffener into a pipe is, therefore, undesirable. Also, most plastic pipe is extruded without precise inside dimension controls. Accordingly, the inside diameter of most standard pipes varies widely and flat spots tend to appear on the interior surface of the pipe along with abrupt changes in surface profile. In small diameter pipe, inside deviations are usually small enough that press or wedge fit stiffeners may work. However, on large diameter pipe the inside diameter variation makes it almost impossible to build a stiffener that fits snug enough to work without being difficult to slip inside the pipe.
Another problem inherent with polyolefin pipe materials is that upon cutting the end of the pipe shrinks by as much as 2% of the diameter. This shrinking is called "toe in". For example, when a 24 inch pipe is cut, the cut end can shrink as much as 0.5 inches in inside and outside diameter. When this happens it is impossible to press or wedge fit a stiffener inside the pipe that has been sized to the original pipe inside diameter.
Thus, there is a need in the art for a method of stiffening flexible pipes which accounts for the various non-uniformities at the ends of a flexible pipe.